Istanbul (Turkish: İstanbul) is Turkey's most populous city as well as its cultural and financial hub. Located on both sides of the Bosphorus, the narrow strait between the Black Sea and the Marmara Sea, Istanbul bridges Asia and Europe both physically and culturally. Istanbul's population is estimated to be between 12 and 19 million people, making it also one of the largest cities in Europe and the world.
Expanding the ancient Roman colony of Byzantium by the order of the Roman Emperor Constantine the Great, the imperial city of Constantinople was for nearly a thousand years the last remaining outpost of the Roman (later termed Eastern Roman or Byzantine) Empire. It was finally conquered by the Ottoman Sultan Mehmed II on 29 May 1453, an event sometimes used to mark the end of the Middle Ages. It was the nerve centre for military campaigns that were to enlarge the Ottoman Empire dramatically. By the mid 1500s, Istanbul, with a population of almost half a million, was a major cultural, political, and commercial centre. Ottoman rule continued until it was defeated in WWI and Istanbul was occupied by the allies. When the Republic of Turkey was born in 1923 after the War of Independence, Kemal Atatürk moved its capital to the city of Ankara. However, Istanbul has continued to expand dramatically; today its population is approximately 14 million and increases at an estimated 400,000 immigrants per year. Industry has expanded even as tourism has grown.